The measurement of refractive indices of fluids with the aid of two-beam interferometers has been known for some time. The two-beam interferometers base the measurement of the refractive index on a comparison of the optical wavelengths in two cells separated from one another and which are geometrically identical. The substance to be investigated is in one cell and another substance with a known refractive index is in the other cell. In lieu of another substance, a vacuum can be established in the other cell.
In most interferometers of this kind, two fringe systems are generated by means of two separate but like beam paths of which the one beam path does not pass through the object to be measured and therefore remains unchanged during the measurement and thus serves as a readout index for the other fringe system which becomes changed by the measurement. Interferometers with two fringe systems afford the advantage that they are substantially insensitive to influences of temperature and mechanical deformation of the apparatus.
The refractive index is determined in that the excursion of the fringe system is either read off a scale or in that the fringes are shifted again into the zero position by means of an optical compensator in the beam path of the measuring cell whereby the magnitude of the displacement provides the measured value. Interferometers of this type are described, for example, in German Pat. No. 10 22 032 or in German published patent application DE-OS 25 07 183.
German Pat. No. 23 06 091 describes an interference refractometer wherein the measurement and reference cells are configured as separate Fabry-Perot interferometers having lengths which are periodically changed with the aid of a common electro-strictive apparatus. Both cells have curved interference mirrors of spherical shape with a high reflectivity so that the light radiated into the mirror exits only after a great many reflections. When passing through the region of the electro-strictive apparatus, sharply limited resonance transmittances occur with monochromatic light which are sharply defined with respect to comparatively wide ranges in which the cells are opaque. If a medium having a changed index of refraction gets into the measuring cell, the resonance frequencies of the measuring cell become displaced with respect to the resonance frequencies of the reference cell via the change of the optical path length. The magnitude of the displacement is a measure of the change of the index of refraction. Disturbing influences which change the resonance frequencies of both cells in the same manner do not affect the measurement.
With the above-described apparatus, the determination of the refractive index in dependence upon wavelength is only possible, if at all, by making individual measurements at various wavelengths which is complex and takes an inordinate amount of time.
German Pat. No. 21 53 315 discloses an interference spectral photometer wherein the spectral characteristic of the refractive index or the transmittance of a specimen is determined. In this arrangement, the light beam emanating from a continuous radiator is divided into two component light beams which are intensity modulated at different frequencies. The two component beams pass through an interferometer arrangement parallel to one another and each component beam is split up into two subcomponent beams and again united and conducted to a common beam receiver. The interferometer arrangement includes a scanning mirror for adjusting changes of the optical path differences of one interferometer branch. From an evaluation of the receiver signal, the interferometer arrangement simultaneously delivers the specimen interferogram and the background interferogram because of the two component beams. The spectral characteristic of the refractive index or the transmittance of the specimen is obtained with the aid of a suitable computer from the specimen and background interferograms.
Practical applications of this arrangement for measuring the refractive index are not known and this is not surprising because the dispersion in the spectral range of 5 to 500 .mu.m specified for the arrangement is not especially interesting. Furthermore, the arrangement is also not suitable for measurements where intense absorption is present.
Finally, it is known from the literature that the refractive index can be determined from the interference superpositions with transmission spectrums or reflection spectrums. In spectroscopy with known indices of refraction, the precise layer thickness is determined mostly from the interference superpositions. However, work also is known wherein with the layer thickness known, the refractive index is determined. Thus, for example, an article entitled "The Index of Refraction of Germanium Measured by an Interference Method" by D. H. Rank et al in the Journal of the Optical Society of America, Volume 44, Number 1, (January, 1954), pages 13 to 16, describes that the refractive index of a germanium crystal having a thickness of 3 mm is determined in the wavelength range of 2.0 to 2.4 .mu.m by evaluating the interference superpositions of the transmission spectrum. An article entitled "Determination of Refractive Index and Film Thickness from Interference Fringes" by N. J. Harrik in Applied Optics, Volume 10, Number 10, (October 1971), pages 2344 to 2349, discloses that the refractive index in the range of 2.5 to 7.0 .mu.m as well as the thickness of films are determined by measuring the reflective spectrum at various angles of incidence.
The known arrangements are not suitable for making measurements on very small volumes of fluid as is the situation, for example, with high-pressure fluid chromatography and especially with micro high-pressure fluid chromatography. In this area, there is also the requirement to be able to measure into the ultraviolet range. The known arrangements are further poorly suited or not at all suitable where intense absorption is present and for specimens for which the danger of a thermal or photochemical decomposition is present because of the radiation energy for making measurements which is directed to the specimen.